1. Field of the Invention
The present invention relates to an integrated-circuit power amplifier of a bridge transformer less (BTL) type having a standby switch which is particularly used in audio systems.
2. Description of the Related Art
Recently, most of integrated-circuit power amplifiers used in audio systems have a BTL structure as shown in FIG. 1 in order to achieve high power. In the BTL structure, output voltages V.sub.1 and V.sub.2 of amplifier circuits 101 and 102 have phases opposite to each other and the same amplitude. Voltage V applied to load 103 such as a speaker is thus expressed as follows. EQU V=V.sub.1 -V.sub.2 =V.sub.1 -(-V.sub.1)=2V.sub.1
where V.sub.1 =-V.sub.2. The amplitude in the voltage applied to load 103 is about twice as large as the ordinary amplitude. When the amplitudes of output voltages V.sub.1 and V.sub.2 of amplifier circuits 101 and 102 are increased in the same phase, no output waveform appears, accordingly no voltage is applied to load 103.
The BTL type power amplifier usually includes a standby switch for not directly turning on or off a power source but turning on or off an internal transistor circuit and cutting off a circuit such as a bias source to equivalently turn on or off amplifier circuits 101 and 102. Any large-capacity relay or switch is not therefore required, and amplifier circuits 101 and 102 can be turned on or off by a small-power switch or a signal supplied from a microcomputer.
Further, in the BTL type power amplifier, it is necessary to eliminate noise (i.e., pop) caused in the transition from the on-state to the standby state (in which the power source is on but the bias source is off and the amplifier circuits are equivalently in the offstate) of amplifier circuits 101 and 102 and to avoid a noise-remaining phenomenon in which the noise remains for a short period of time after the on-state has been changed to the standby state by the time constant of an external capacity. In the transition, therefore, the bias source is not turned off instantaneously but after the amplitudes of output voltages V.sub.1 and V.sub.2 of amplifier circuits 101 and 102 are quickly increased in the same phase so as to reach the power source voltage or ground voltage (or negative power source voltage) and then the output signals of the amplifier circuits are extinguished.
The BTL type power amplifier having the above structure is used as an audio mute for eliminating noise caused when a tuner is detuned. If the power amplifier malfunctions, its on-state is changed to the standby state to prevent noise from generating from speakers. It is more effective when a system such as tuner and amplifier malfunctions earlier than the power amplifier itself by a sudden change of the power source voltage as in audio systems installed in automobiles. More specifically, when a power source (Vcc) line is monitored and its voltage is dropped to such an extent that the system cannot be normally operated, a standby signal is supplied to the power amplifier. If a microcomputer includes a low-voltage detecting function, a logic signal is supplied to the power amplifier to render it in the standby state.
When the power source voltage is dropped even in the standby state, a battery line may be cut off by the main key switch of, for example, an audio system installed in an automobile. In this state a circuit for maintaining the standby state cannot be normally operated and a pop may thus be generated in the output signals of the amplifier circuits.
The detection of a decrease in the power source voltage using a power-source line monitor arranged at the power amplifier side is very difficult since a detected voltage and detection timing vary from system to system. If the bias source is instantaneously cut off in the transition from the on-state to the standby state, a pop may be generated by the influence of the external capacity.